A spark plug is a component which generates spark discharge in order to ignite an air-fuel mixture inside a combustion chamber. As a structure of the spark plug, there is known a structure which includes an insulator internally having an axial hole extending along an axis of the insulator, a metallic shell for internally holding the insulator, a center electrode held inside the axial hole, and an electrically conductive seal for holding the center electrode inside the axial hole (refer to Patent Document 1). In the case of the structure disclosed in Patent Document 1, the center electrode includes a flange portion which bulges in a radial direction, and a head portion which protrudes rearward from the flange portion. This structure is utilized so as to hold the center electrode in the insulator. Specifically, the flange portion is brought into contact with a step portion provided in the axial hole, thereby restraining forward movement of the center electrode. Furthermore, a space around the head portion and the flange portion is filled with a seal so as to ensure the impact resistance of the center electrode. In this manner, even if the center electrode receives an impact due to the combustion, the center electrode is less likely to loosen.